Formation of chromium alloy coatings



April 12, 1932-. H. MARSHALL" 7 1,853,369

FORMATION OF CHROMIUM ALLOY COATINGS Eiled Dec. 27. 1927 Arramwry It is well recognized Patented Apr. 12, 1 932 UNITEDII-STATES PATENT OFFICE LESLIE H. MARSHALL, OF COLUMBUS, OHIO, AS SIGNOR TO THE-TECHNIMET COMPANY, OF COLUMBUS, OHIO, A CORPORATION OF OHIO "FORMATION OF CHROMIUM ALLOY COATINGS Application filed December 27, 1927. Serial 1T0. 242,887.

The invention relates to the production of a chromium alloy coating or layer on the surface of articles to which it is desired to give superficially the characteristics of metallic chromium or its alloys.

that chromium and many of its alloys have properties that are desirable, such as susceptibility of taking a high polish with resultant pleasing appearance, resistance to various kin'ds of corrosion, resistance to oxidation at high temperatures,

and remarkable-hardness. Articles made of 'chromium alloys have accordingly had a certain limited use. These alloys, however, are relatively expensive and somewhat diflicult to form into more or less complicated shapes; and these considerations limit the commercial use of the chromium alloys. Where the pur pose is to secure only the surface characteristics of chromium or its alloys, such as resistance to corrosion, pleasing appearance or superficial hardness, it has been realized that an alloy surface layer upon the article would serve t'e purpose practically as well as an article composed entirely of the alloy. It has, accordingly, been sought to give to articles superficial chromium alloy surfaces but, as far as Iam aware, none of the proposals so far advanced have been susceptible of. any considerable commercial application because of the relativelyhigh temperatures involved in the treatments employed and the great difficulty of avoiding oxidation both of the chromium and ofthe surface to be coated or alloyed.

One of the principal objects of the present invention is the provision of a process for forming chromium alloy coatings which inarticle treated are rendered clean by posi-,

tive fluxing action with resultant uniform coating thereof.

My improved process, generally considered,

involves enclosing the article to be given the chromium alloy coating with material which will, when heat. is applied thereto, provide (a) a non-oxidizing atmospherd capable of displacing air from the space surrounding the article before the temperature is high enough to cause substantial oxidation of the article to be coated, and/or (12) a fluxing actionon the article to be coated insuring clean surfaces thereof, to the end above noted, and (c) a compound of chromium adapted to vaporize at least in part by the time the moderate temperature of the treatment is attained, and further adapted, while in the vapor state, to react or dissociate with resultant deposition of chromium on the-article treated and the formation of a chromium alloy surface thereon. v

More specifically, in my preferred procedure, the material with which the article to be treated is enclosed, includes either metallic chromium or an alloy or a compound thereof, and a substance capable of convervmetal or ,other article to be treated oxidize to any substantial extent, and also capable in the vapor or gaseous state of reacting with the chromium or its compound to form a chromium compound which in turn takes the vapor state at the temperature of the treatment and in that state reacts or dissociates with the resultant deposit of chromium upon the article to form an alloy coating thereon.

In order that my invention and the method of practicing thesame may be clearly understood, concrete examples will now be given, reference beinghad to the accompanying drawing which shows more or less diagrammatically a simple form of apparatus suitable for the carrying out of the process.

Let it be assumed, for example, that the articles to be coated are formed of steel with carbon content of about 0.2%. These articles, designated by the numeral 1 in the drawtoo ing, after being thoroughly cleaned by' sand blasting are introduced into a suitable container 2 and imbedded, as shown, in a packing 3 made up of a mixture of crushed ferro chromium (70% chromium, 0.1% carbon), similarly crushed chrome ore to the amount of twice the volume of the crushed ferrochromium and dry bleaching powder (chlo.- ride of lime) to the amount of 5% by weight of the entire packing. Preferably the ferrochromium and chrome ore are crushed to pass a six mesh-screen, being of varying particle sizes up to the maximum size capable of passing such a screen. As will be noted, the container has a tightly closed bottom so that vapors or gases cannot escape. Also, a layer of packing material of considerable thickness is covered over the topmost articles to be treated to protect them from gases that might enter through the top of the receptacle. However, the articles are further protected from the entrance of gases by the provision of a closely fitting cover 4, which is sealed at the edges by a luting 5 of refractory material, such as powdered flint, mixed with water to form a paste. The container 2 may preferably be made of low carbon, high chromium (25% chromium) iron, which is adapted to resist oxidation and deformation at high temperatures. The container and contents are now placed in a furnace and heated to 1900 F. and held at that temperature for twenty hours. moved from the furnace and allowed to cool to room temperature. i

After the container and contents have cooled, the treated articles are removed from the container and packing and immediately washed in hot water and then boiled in a 10% sodium carbonate solution for one-half hour to neutralize and remove any salts that might remain on their surface, the chromium-iron alloys being susceptibleto attack by the halide salts to some extent. The treated are then washed and dried.

The articles thus treated are found to have a coating bright in color and about 0.001

articles inch thick, the coating consisting of a high percentage of chromium at its outer surface with the percentage of chromium decreasing toward the inner part thereof.

The procedure above described may, within the scope of the present invention, be varied considerably with respect to materials employed, time of treatment, temperature of treatment and in other respects. Thus the material supplying the chromium may'take various forms. For example, chromium metal (98% chromium) crushed to pass a two mesh screen and be retained on a twelve meshs'crecn, and mixed with 5% of its weight *of bleaching powder. but not diluted with chrome ore or other diluent, will give a bright and shiny coating of chromium on low carbon steel articles after treatment at 1800 F. for

The. container is then repasses the screen along with the coarse, gives five hours. Again, ferro-chromium chromium, 0.1% carbon) crushed to pass a six mesh screen and mixed with 5%' of its weight of bleaching powder but not otherwise diluted will give a good coating on low carbon steel articles after a treatment at 1800 F. for ten hours. Furthermore, diluents such as carbon, silicon, or other elements may be present in the chromium or ferrochromium used. However, when a substantial amount of silicon or other element capable of reacting with the non-oxidizing gas or vapor and of alloying with the base metal and the chromium is present, the composition of the alloy coating formed will be modified accordingly. As indicated by the examples given above, the packing material may be otherwise diluted, with consequent decrease of the rate of coating. Examples of such other diluents inaddition to chrome ore, are sand, alumina, magnesia and other refractory materials. These diluents, it may be noted, reduce a tendency ,of the packing material to cake around the articles treated, though in the case of sand, particularly when carbon is present, there may be some reduction of the sand with the resultant'caking of the packing material, It is noted further that the particle size ofthe crushed chromium packing material is a matter of some practical importance. Pulverizcd material naturally presents a larger surface and yields a coating somewhat more quickly than the preferred six mesh material but the cost of pulverizing the material is relatively high and the increased expense is not warranted by the relatively small increase in the coating rate. Also, in the case of low carbon ferrochromium in particular, the material, if finely divided, tends to stick to the surface of the article treated and gives a less attractive appearance. On the whole, the chromium or ferro-chromium when crushed to pass a six mesh screen, including the fine material that the preferable results for general purposes.

The material used as a flux and chromium carrier should be volatile at the temperature of treatment, and preferably, as previously indicated, it should volatilize at below the temperature at which either the packing material or the article to be coated oxidizes-materially. I have found halogen compounds that volatilize or at least have an appreciable vapor pressure, below 700 F. to be satisfactory. Some of these compounds merely volatilize at temperatures below 700 F. and thus displace any air present in the container, the vapors being heavier than the air. Examples of halogen compounds that volatilize, at least in part, below 700 F. are aluminum chloride, antimony chloride, arsenic chloride, mercuric chloride, phosphorous trichloride, carbon tetrachloride, sulphur monochloride and bismuth chloride. 130

Ammonium chloride also vaporizes below 700 F. and within the same temperature range dissociates with formationof ammonia and hydrochloric acid, both of which are gaseous and adapted to displace air from the treating container. Also, the hydrochloric acid has an etching effect on iron and gives a clean surface, and at the higher temperatures of the coating treatment it dissociates to permit some chlorine to react to form chromium chloride. Others of the halogen compounds liberate halogens on heatin therefore particularly 'suita le. Examples of this latter class are gold (auric) chloride, ferric chloride, cupric chloride, phosphorous pentachloride, sulphur dichloride, sulphur tetrachloride and bleaching powder. Of the halogen compounds mentioned, bleaching powder, aluminum chloride and ferric chloride are probably most suitable for my process. Thus, in the foregoing specific example the bleaching powder can be replaced by a similar amount of ferric chloride (preferably though not necessarily anhydrous) or of aluminum chloride. The ammonium chloride is not so desirable for my purposes be- 7 cause the ammonia evolved tends to react objectionably with iron being coated. Halogen compounds other than chlorides are operable in my process; thus 5% of anhydrous ferric bromide can be used in place of the bleaching powder given in the above example, though the bleaching powder is preferable because of its lower cost. compound employed need not be introduced as such into the treating container. Thus in the packing material there can be substituted for the 5% of ferric chloride, for example, 10% (by weight) of a mixture of equal amounts by weight of powdered ferrous sulphate (preferably anhydrous) and powdered sodium chloride. On heating the packing, these two compounds react to give ferric chloride and sodium sulphate. Any other known method of formin ferric chloride, by

reaction within the packing, might be used. However, this method of introducing the ferric chloride usually contaminates the packing, as with the sodium sulphate in the'exal nple just given, and this is ordinarily undes1rable.

It should 'be pointed out that chromium chloride has a low vapor pressure at the temperatures employed in my process and consequently in carrying out the process the chromium present in the treating chamber should be in correspondingly close proximity to the casting to insure contact of the chromium chloride with the casting.-

The base metal on which the coating is produced, is not limited to iron and steel alone, but malleable cast iron, gray cast iron. and ferrous alloys can be coated; also other metals such as nickel, molybdenum, and tungsten can be coated. Thus nickel can be and they are Furthermore, the halogencoated in the same way and under the same conditions as the low carbon steel objects cited above.

Similarly, the temperature of the treatment may be varied substantially. This variation depends both on the duration of the heat treatment and on the packin material used. The nature of the article to e coated also has some small effect, since iron very low in carbon coats at a slightly faster rate than does steel. Thus commercially pure chromium, 5% carbon) about ten times as long. 7

The duration and temperature of the treatment are closely related and interdependent.

.Thus the undiluted low carbon 70% ferrochromium will give a good coating on low carbon steel in eight hours at 1800 F.; but by extending the treatment to a period of sixty hours the temperature can be lowered to 1600 F., and if chromium metal is used in place of the ferro-chromium, the tempera ture for the sixtyhours treatment can be reduced to about 1500 F. On the other hand, quite short periods of treatment are sufiicient at a temperature of 2000 F., but the difiiculties of working at this temperature increase the production cost and limit commercial use of such temperatures. Furthermore, it is to be noted that for steel articles or other ferrous articles containing 0.8% or more of carbonthe temperature of treatment should be kept below 1800 F. to avoid danger of burnin the steel, i. e. destroying its strength by incipient fusion. This danger lis not present in the case of low carbon stee While I am unable to say with certainty precisely what goes on in the carrying out- -of my process, as at present advised I believe that the following actions occur. In the first stages of the heating, i. e. while the temperature is rising to say 7 00 F., the bleaching powder or other halogen compound employed vaporizes, dissociates with evolution of chlorine (or other halogen), or both of these actions occur. In any case, the vapor or halogen gas formed displaces any air presthe heating continues the halogen compound in the vapor state, or the chlorine or'other halogen evolved, reacts with the chromium'to form chromium .chloride. Thus, bleaching powder when heated evolves chlorine by sociation as follows:

CaOCl=CaO +01 The chlorine thus liberated reacts with the chromium. as stated to form chromium chloride and the latter in the vapor state penetrating all parts of the space within the container reacts or dissociates in the presence of the article of iron or other material to be coated, and deposits metallic chromlum. thereon.

- The deposited chromium at the temperature of treatment penetrates the surface zone of the iron article and alloys therewith so that the coating becomes in effect an integral part of the article. The chlorine might, of course, in part react with the iron of the article being coated to yield ferrous chloride, but the latter would to some extent dissociate and thus make the chlorine again available for the con tinuation of the chromium coating process. Whatever the precise actions may be that go on during the process, the result is the for- .mat-ion of a complete and highly. uniform adapted to many commercial uses. Thisre 'sult is secured, furthermore, with a process .Which lends itself in a remarkable degree to 1 production on a commercial basis. In-particular, the prevention of oxidation by the. simple expedient of introducing into the' treating chamber a substance adapted to vaporiz'e or evol'vg 1a gas capable of displacing air from the treating space is accomplished I with the greatest case under working conditions readily attainable, The articles to be coated are packed in the treating container under ordinary conditions without exclusion of air, difliculties that would be incident to the introduction into the treating container of a non-oxidizing gas as such are avoided, approximately atmospheric pressure is used throughout the process, and where thehalide of the metal to be coated is used as the flux -orcarrier further advantage is attained.

Again, the conversion of the chromium into a compound vaporizable at moderate temperatures and capable of reacting or dissociat-' ing in the presence of the article to be coated with resultant deposition of chromium, both makes the process'susceptible of operation under working conditions that can'be maintained on a commercial basis without difliculty, and also renders the process available for work that could-not be accomplished at higher temperatures. For example, such high temperatures are not feasible in the case of the high carbon steels, as "above pointed out. Again, the moderate temperature which suflices in my process makes the process available for the coating of malleable iron castv ings, whereas the high temperatures whichhave characterized earlier proposals for chromium coating would destroy the qualities of such castin s. In a separate application Se-" rial No. 41 ,97 8, filed January-2, 1930, I set forth in detail the application of the present process to the combined malleableizing and coating of iron castings.

In characterizing herein the gas or vapor in the treating container as non-oxidizing,

that term is, of course, used in its strict or limited sense indicating that the gas or vapor is of such a nature that it will not form a film of metallic oxides on the article being coated.

While I have set forth somd of the varia-- tions of materials, temperatures and time of treatment which may be used in the carrying out of my invention, it should be understood that I have not attempted to point out all variations of this character but that the invention comprehends all modifications within the scope of the appended claims.

That I claim is: 1. The process of forming alloy coatings of chromiumv which includes the steps of surrounding the article to be coated with a packingcontaining chromium or an alloy of chr0- mium and the metal of which the said article is constituted; applying heat to the said article and packing; permeating the space surrounding the article with suflicient halogen or halogen compound gas or vapor before the temperature has been raised above the oxidizing' temperature of the reacting metals present substantially to displace the air from said space and thereby prevent oxidation of the said metals; and thereafter continuing the. heatingof the packing and article to temperatures between 1500 F. and 2000 F. in the presence of halogen or halogen compound gas or vapor until aacoating of chromiumalloy is formed on the article.

2. The process of forming alloy coatings of chromium which includes the steps of surrounding the article'to be coated with a packing containing chromium or an alloy of chromium and the metal of which the said article is constituted; applying heat to the said article and packing; permeating the space surrounding the article with sufiicienthalogen or halogen compound gas or vapor before the temperature has been raised above the oxidizing temperature of the reacting metals present substantially to displace the air from said space and thereby prevent oxidation of the said metals; and thereafter continuing the heating of the packin and article to temperatures between 1500 i and 2000 F. in the presence of halogen or halogen compound gas or vapor until the halogen or halogen compound gas or vapor reacts with the chromium to form a chromium compound and the latter reacts or dissociates in the presence of they article to form a coating of chromium alloy thereon.

3. The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing chromium or an alloy of chromium and the metal of which the said article is constituted and a halogen or halogen compound adapted to evolve a non-oxidizing gas or vapor at temperatures below the oxidizing temperature of the reacting metals present; heating the packingand contained article to cause such evolution of gas or vapor in sulficient amount to permeate the s ace surrounding the article, substantially displace the air from said space and thereby prevent oxidation of the said metals; and further heating the packing and article to temperatures be tween 1500.F. and 20Q0 F. in the presence of halogen or halogen compound gas or vapor until a coating of chromium alloy is formed on the article.

4:- The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing chromium or an alloy of chromium and the metal of which the said article is constituted and a material adapted to evolve a halogen gas at temperatures below the oxidizing temperature of the reacting metals present; heating the packing and contained article to cause such evolution of halogen gas in sufiicient amount to permeate the spacesurrounding the article, substantially displace the airfrom said space and thereby prevent oxidation of the said metals; and further heating the packing and article to temperatures between 1500 F. and 2000 F. in the presence of halogen or halogen compound gas or vapor until a coating of chromium alloy is formed on thearticle.

5. The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing chromium or' an alloy of chromium and the metal of which the said article is constituted, an inert diluent and a halogen or halogen compound adapted to evolve nonoxidizin gas or vapor at temperatures below the oxidizing temperature of the reacting metals present; heating the packing and contained article to cause such evolutlon of gas or vapor in an amount sufficient topermeate the space surrounding the article, substantially displace the air from said space and thereby prevent oxidation of the said metals; and further heating the packing and article to temperatures between 1500 F. and 2000 pound as or vapor until a coating of chromium a loy is formed on the article.

6. The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing chromium or an alloy of chromium and the metal of which the said article is constituted; applying heat to the said article and packing; 'permeatin the space surrounding the article with su cient halogen or halogen compound gas or vapor before the temperature has been raised above the oxidizing temperature of the reacting metals present substantially to displace the air from said space and thereby prevent oxidation of the said metals; thereafter continuing the heating of the packing and article to temperatures etween 1500 F. and 2000 F in the presence of halogen or halogen compound gas or vapor until a coating of chrw mium alloy is formed on the article; removing the article from the packing; and removing from the surface of the article adherent halide salts. V

7 The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing ferro-chromium and a halogen or halogen compound adapted to evolve a non-oxidizing gas or vapor at temperatures below the oxidizing temperature of the reacting metals present; heating the packing and contained article to cause such evolution or gas or vapor in an amount suflicient to permeate the space surrounding the article, substantially displace the air from said space and thereby prevent oxidation of the said metals; and further heating the packing and article to temperatures between 1600 F. and

2000 F. in the presence of halogen or halogen compound gas or vapor until a coating of chromium alloy is formed on the article.

8. The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing chromium or an alloy of chromium and the metal of which the said article is constituted and bleaching powder; heating the packing and contained article to evolve chlorine from the bleaching powder in an amount sufiicient substantially to displace air from the space surrounding the article and thereby prevent oxidation of the said metals; and further heating the packing and article to temperatures between 1500 F. and 2000 F. in the presence of halogen or halo en compound gas or vapor until a coating 0 chromium alloy is formed on the article.

9. The process of forming alloy coatings of chromium which includes the steps of surrounding the article-to be coated with a packin g containing chromium or an alloy of chro-.. mium and the .metal of which the said article is constituted and a halogen or halogen com;

gas or vapor at temperatures below the oxidizing temperature of the reacting metals present,

said packing being held in a container with tight side and bottom walls; heating the packing and contained article to cause such evolution of gas or vapor in an amount suf-' ficient substantially to displace air from the space surrounding the article and thereby -prevent oxidation of the said metals; and

further heating the packing and article to temperatures between 1500 F. and 2000 F. in the presence of halogen or halogen compound gas or vapor until a coating of chromium alloy isformed on the said article.

10. The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing chromium or an alloy of chromium and the-metal of which the said article is constituted and a halogen or halogen compound adapted to evolve a non-oxidizing gas or vapor at temperatures below 700 F. heating the packing and contained article to cause such evolution of'gas or vapor in an amount 1 suflicient substantially to displace the air from the space surrounding the article and thereby prevent oxidation of the reacting metals present; and further heating the packing and article to higher temperatures in the presence of halogenor halogen compound gas or vapor until a coating of chromium alloy is formed on the article.

11. The process of forming alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing chromium or an alloy of chromium and the metal of which the said article is constituted and a halogen or halogen compound adapted to evolve a non-oxidizin gas or vapor at temperatures below the oxidlzing temperature of the reacting metals present; heating the packing and contained article to cause such evolution of gas or vapor in an amount sufiiciently substantially to displace the air from the space around the article and thereby prevent oxidation of the said metals; and further heating the packing and article to tem ratures above the oxidizing temperature o the metals present to cause (a) reaction between the gas or vapor evolved by the halogen compound and the chromium to form v a halide of chromium, (6) some vaporization of the said halide of chromium and-(c) reaction or dissociation of the halide of chromium with the resultant deposition of chromium on the article to be coated.

12. The process of forming'alloy coatings of chromium which includes the steps of surrounding the article to be coated with a packing containing'chromium or an alloy of chropresent; heating the packing and contained article to cause such evolution of gas in an amount suflicient substantially to displace the air from the space around the article and thereby revent oxidation of the said metal; and er heating the packing and article to temperatures'between 1500 F. and 2000 mium or ferro-chromium and a halogen or halogen compound-adapted to evolve a nonoxidizing gas or vapor at temperatures below the oxidizing temperature of the reacting metals present; heating the packing and contained article to cause such evolution of gas or vapor in an amount sufiicient substantially to displace the air from the space around the article and thereby prevent oxidation of the said metals; and further heatin the packing and article in the presence 0 halogen or halogen compound gas or vapor to higher temperatures, but not to exceed 1800 F. in the case of the article, until a coating of chromiumv alloy is formed on the article.

14. The process ofv forming alloy coatings of chromium on ferrous articles containing 0.8% or upwards of carbon, which process includes the steps of surrounding the article to be coated with-a packing containing chromium or ferro-chromium and a halogen or halogen compound adapted to evolve a nonoxidizing gas or vapor at temperatures below the oxidizing temperature of the reacting metals present; heating the packing and contained article to cause-such evolution of gas or vapor in an amount suflicient substantially to displace the air from the space around the article and thereb revent oxidation of the said metals; an iiirther heating the packing and article in the presence of halogen or halogen compound gas or vapor to temperatures above the oxidlzing temperature of the said metals, but not to exceed 1800 F. in the case of vthe article, until a coating of chromium alloy is formed on the article.

' 15. The process of forming alloy coatings of chromium which includes thesteps of surrounding the article to'be coated with a packcontaining chromium or an alloy of chronnum and the metal of which the article is constituted and a halide adapted to evolve a halogen gas at temperatures below the oxidizing temperature of the reacting metals present; heating the packing and contained articleto cause such evolution of gas in an amount sufiicient substantially to displace the air from the space around the article and thereby prevent oxidation of the said metals;

and further heating the packing and article in the presence of halogen or halogen compound gas or vapor to temperatures above the oxidizing temperature of the said metals until a coating of chromium alloy is formed on v the article.

16. The process of forming alloy coatings of chromium on ferrous articlesvwhich includcs the steps of surrounding the ferrousarticle to be coated with a packing contain-- ing chromium or ferro-chromium and a halide adapted to evolve a halogen gas at temperatures below 700 F.; heating the packing and contained article to cause such evolution of gas in an amount-sufficient substantially to displace the air from the space around the article and thereby prevent oxidation of the reacting metals present; and further heating the packing and article in the presence of halogen or halogen compound gas or vapor to temperatures above the oxidizing tempera ture of thesaid metals until a coating of chromium alloy is formed on the article.

17. The process of forming alloy coatings of chromium which includes the steps of enclosing in a treating chamber the article to be coated together with material including chromium or an alloy of chromium and the metal of which the said article is constituted and with the chromium or chromium alloy in close proximity to the article; applying heat to the said article and material; permeating the space surrounding the article with sufficient halogen or halogen compound gas or vapor before the temperature has been raised above the oxidizing temperature of the react until the halogen orhalogen compound gas or vapor reacts with the chromium to form a chromium compound and the latter reacts or dissociates in the presence of the article to form a coating of chromium alloy thereon. 19. The process of forming alloy coatings of chromium which includes the steps of enclosing in a treating chamber the article to acting metals present substantially to displace the air from said space and thereby prevent oxidation of the said metals; thereafter continuing the heating of the said material and article to temperatures between 1500 F. and 2000 F. in the presence of halogen or halogen compoundgas or vapor until a coating of chromium alloy is formed on the article; and removing from the surface of the article adherent'halide salts.

In testimony whereof, I hereunto aflix my signature.

- LESLIE H. MARSHALL.

ing metals present substantially to displace the air from said space and thereby prevent oxidation of the said metals; and thereafter continuing the heating of the article and the said material to temperatures betweenl500 F. and 2000 F. in the presence of halogen or halogen compound gas or vapor until a coating of chromium alloy is formed on the article. i

18. The process of forming alloy coatings of chromium which includes the steps of enclosing in a treating chamber the article to be coated together with material including chromium or an alloy of chromium and the metal of which the said article is constituted and with the chromium or chromium alloy in close proximity to the article; applying heat to the said article and material; permeat ing the space surrounding the article with sufiicient halogen or halogen compound gas or vapor before the temperature has been raised above the oxidizing temperature of the reacting metals present substantially to displace the air from said space and thereby prevent oxidation of the said metals; and c thereafter continuing the heating of the said material and article to temperatures between 1500 F. and 2000'-F. in the presence of halogen or halogen compound gas or .vapor 

